when i later in that mission abandoned game, and loaded save 111. the H.Explo. worked as expected.
Some went off right away, some went off a round or few later. and some didn't boom.
The error may be narrowed to pre-mission-setup when disarming, and re-arming, then entering mission, throwing, and counting the rounds. as mentioned my Save is 'after' having landed. So opening the BattleScape phase, saving, and immediately reloading that save might be the key to not encountering the issue. But as mentioned...
ALL my H.Exp.'s were set to 0 to start with while landing setup was active.
I disabled them all.
i re-enabled some. ( with timing at 1 so individuals could throw a big boom with enough time for TU drained comrades to escape the radius in the next round. ) and then a bunch with no prearmed status, so i could set accordingly.
Have a look at pre-landing, arm and disarm some H.Exp. see if they show up as armed when you enter BattleScape.
If they DO show up as armed, throw those around, and see if/when they explode. In my live run, the armed ones never did. In my Reload from the save they worked fine. I was actually surprised to see them boom on the reload as they had not on the live run prior.
Either way the fix was for me to load the save from post-landing. (111)